Green grass is living vegetation containing a significant amount of water, which fundamentally alters its interaction with heat and flame. The direct answer to whether green grass burns is that it is highly resistant to ignition under normal circumstances. Its living state makes it an extremely poor fuel source for a self-sustaining flame. However, under specific and intense environmental conditions, this natural resistance can be overcome, leading to fire spread.
The Critical Factor of Moisture Content
The resistance of green grass to burning is governed by the high moisture content held within its cells. This is measured as Fuel Moisture Content (FMC), expressed as a percentage of the plant’s dry weight. Actively growing grass typically maintains an FMC well above 100%, sometimes reaching 300% in fresh growth. This water acts as a heat sink, demanding a large amount of energy before the plant tissue can ignite.
For combustion to occur, the plant material must reach its ignition temperature. This requires pyrolysis, where heat breaks down cellulose and lignin into flammable gases. Any applied heat must first raise the temperature of the water inside the grass and then convert it into steam, which consumes substantial thermal energy. This requirement to evaporate the water before pyrolysis can begin rapidly cools the fuel source, making sustained ignition nearly impossible for typical heat sources like a match or a cigarette butt.
Overcoming Resistance: Conditions for Sustained Ignition
Water acts as a natural fire suppressant within the grass, but this resistance fails when the internal moisture level drops below a specific threshold. For grass fuels, the upper limit for fire propagation is generally considered to be a moisture content of about 30% to 40%. When grass “cures” or dies, its FMC drops significantly, turning it into a readily ignitable fuel source.
Severe drought is the environmental condition most effective at reducing this resistance. Drought causes the plant to lose water internally, even while the foliage retains a green appearance. This lack of available water lowers the energy barrier needed for combustion, allowing the grass to dry to a point where it can support a flame. An intense, concentrated external heat source is also necessary to overcome the remaining moisture barrier.
An intense heat source, such as a hot ember or prolonged exposure to radiant heat, pre-heats the grass immediately ahead of the flame front. The heat dries the moisture out of the plant tissue in seconds, effectively turning the green grass into dry fuel just before the flame reaches it. This pre-heating process bypasses the natural resistance, allowing the grass to reach the necessary pyrolysis temperature and maintain a self-sustaining fire.
Structural Differences in Grass Species
Not all green grass responds identically to heat, as physical structure and overall plant composition introduce variability in flammability. The inherent thinness of grass blades allows for rapid heat transfer and efficient oxygen availability, which can fuel intense and fast-moving fires once ignition is achieved. This high surface-area-to-volume ratio means that once the water is driven off, the remaining material ignites quickly.
The presence of dead plant material beneath the living layer, often termed thatch, significantly increases the fire hazard, even when the surface appears green. This cured material has a low FMC and acts as a readily available fuel bed. Thatch can generate enough heat to pre-dry and ignite the living green growth above it. The overall biomass quantity of a grass species also drives the sustainability of a fire, as species with greater total mass tend to burn more intensely and for longer periods.